with R. W. Helliwell & R. A. Huggins, U.S. Govt. Res. Develop. Report 69 (1969), 158

Previously reported measurements of the diffusion of sodium ions in beta alumina indicated that this material might be a very good ionic conductor at relatively low temperatures. However, the lack of a suitable nonpolarizing electrode material has prevented the accurate determination of the magnitude of the conductivity to date. It is the purpose of this paper to report a series of measurements using sodium tungsten bronzes as a new type of solid electrolyte that is reversible to sodium ions and applicable over a wide range of temperature at low oxygen partial pressures. The transport of sodium ions takes place by an interstitialcy mechanism. [Received 31.7.1970, published 1 January 1971]

with Robert A. Huggins, ‘Electrochemical preparation and characterization of alkali metal tungsten bronzes, M_xWO₃', in Solid State Chemistry – Proceedings of the 5^th Materials Research Symposium sponsored by the Institute for Materials Research, National Bureau of Standards, October 18-21, 1971 held at Gaithersburg, Maryland, edited by Robert S. Roth & Samuel J. Schneider Jr., NBS Special Publication 364, Washington DC: U.S. Department of Commerce and National Bureau of Standards, 1972, 51-62

The tungsten bronzes are nonstoichiometric compounds of general formula MxWO3 where M is usually a monovalent cation and x is in the range 0 to 1. They are of much interest particularly because of their exceptionally wide stoichiometry ranges. Although discovered originally by Wöhler in 1824, there is still today no published thermodynamic data on any tungsten bronze system. It is the purpose of this paper to discuss the thermodynamic aspects of the processes occurring during the electrolytic decomposition of tungsten melts to form sodium and potassium tungsten bronzes. [Issued July 1972]

 

with Robert A. Huggins, ‘Beta alumina – Prelude to a revolution in solid state electrochemistry', ibid., 139-154

Interest in beta alumina was sparked off by pioneering work at the Ford Motor Company with a revolutionary approach to the battery problem: using liquid electrodes and a solid electrolyte.

By use of a group of novel non-polarizing solid electrodes, which are fully reversible to the monovalent cation, it has been possible to study the ionic conductivity of a series of beta aluminas containing alkali metals as well as thallium over a wide temperature range. The electronic conductivity of silver beta alumina has also been determined by use of the Wagner asymmetric polarization technique over wide ranges of temperature and oxygen partial pressure.

 in Conf. Layered Mater., Monterey, California, August 1972? [not found in MIT library catalogue]

with Robert A. Huggins, in Reactivity of Solids, edited by J. S. Anderson et al, London: Chapman & Hall, 1972, 125 [bestilt fra RSC, ankommer 3. januar 2001]

with P. S. Connell & R. A. Huggins, Jn. Solid State Chemistry, 5 (1972), 321

In a search for materials exhibiting fast ion transport for use as the electrolyte or eledctrode component of solid state battery systems, one needs both a rapid search tool to give an indication of whether ions move and a subsequent technique to accurately determine the diffusivities of the mobile species.

Two methods that are particularly applicable to a rapid survey are ion exchange and nuclear magnetic resonance line-narrowing. For accurate determination of fast ion transport in solids the most satisfactory technique is the direct measurement of the ionic conductivity. However, this is very difficult to accomplish when the ions move rapidly because of interfacial polarisation; a new technique involves the use of solid non-stoichiometric compounds reversible to both the mobile specie and to electrons is discussed.

The advantages and disadvantages of each method will be compared and contrasted relative to its use as a screening or quantitative tool and the properties of the materials under test.

The lithium intercalation compounds of the layered transition metal dichalcogenides, prepared by reaction with n-butyl lithium, have been characterized by x-ray analysis. The group IVB and VB compounds (prepared this way) are stoichiometric and stable; the intercalation proceeds by simple expansion of the lattice at the Van der Waals gap. The group VIB chalcogenides, SnS2 and ReSe2 do not form lithium intercalation compounds with the exception of MoS2 and that appears to be metastable.

(Much of the recent effort on these materials has been aimed at understanding their superconducting properties. These properties are markedly changed on intercalation, thus the superconducting transition temperature is raised from 0.8K of 2H-TaS2 to 4.2 and 5.3 on intercalation of ammonia and hydrated potassium. To date the highest transition temperatures have been observed in the alkali metal complexes for both TaS2 and MoS2.)

[Received 14.3.1975, published 1975]

Currently, high energy density lithium storage systems are the focus of intense interest and expectation. In order to scan and characterize oxidants as potential cathodes, it would be very useful to have a reliable chemical agent which could, in effect, mimic the half-cell reaction under consideration. Although reagents formed by dissolving alkali metals in ammonia, polyamines, polynuclear aromatic solutions in ethers, or hexamethyl phosphoramide might be used, they have a number of shortcomings. Their reactions are difficult to run, since they involve extremely reactive materials, very dark opaque media, their polar solvents can also partake in the reaction, and it is difficult to separate the product from residual reagent. It would be preferable to have instead an agent whose lithium activity is more on the order of a volt or so below the alkali itself. We report that n-buytl lithium serves as such an agent. [Ms. submitted 21.3.1977, revised 11.4.1977, published September 1977]

MSW, in McIntyre, Srinivasan, Will (eds.), Proc. Symp. Electrode Materials and Processes for Energy Conversion and Storage, 784

Although alkali metal batteries have been studied for many decades, it is only recently that the critical importance of the formation of ternary compounds for cell reversibility has been recognized. Much work was first aimed at the tungsten and vanadium bronzes, MxWO3 and MxV2O5, as cathodes, but the diffusivity of the alkali ions was too low. The key requirements for a useful cathode material forming a ternary phase with alkali metal ions are:

·         A high free energy of formation

TMD perform well as cathodes. Transition metal trisulfides also exhibit electrochemical activity. Nickel phosphorus trisulfide will react with more than four lithium resulting in a cell with a theoretical energy density double that of TiS2. If this high theoretical energy density is truly electrochemically reversible then the low cost of the component elements, the reasonable conductivity of the iron and nickel compounds, and the ambient temperature operation, will make the Li-MPS3 batteries promising candidates for electric vehicle propulsion (patent applied for). [Received 31.5.1977, published 1977]

1978

NMR is a particularly flexible tool that can provide valuable information about the degree of ionization, environment, and motional characteristics of the constituent nuclei. It may be applied to single crystals or powdered samples of either insulating or conducting materials. Both short- and long-range motion can be examined. Inequivalent sets of atoms can often be differentiated. However, because it is difficult in simple NMR studies to separate local and extended motion and since they are not sensitive to interfacial affects, NMR observations should be complemented by conductivity studies. However, the absence of motional effects on NMR is a very strong indication of low diffusion. The simplest screening technique is to measure the linewidth as a function of temperature.

‘Chemistry of intercalation compounds: metal guests in chalcogenide hosts', Progress in Solid State Chemistry, 12 (1978), 41-99

To date, only the dichalcogenides have conclusively been shown to form true intercalation compounds, that is compounds where the host may be recovered in its original form on removal of the guest species. Much effort is likely to be spent on the oxide compounds in the next few years both to understand their insertion compounds and to find those that might be applicable in electrodes for energy storage or display purposes. A few have been identified here, and others that fulfill the following three criteria are the most likely candidates. Structures that:

·         are layered or have accessible tunnel sites

·         have an accessible electronic band structure and high energy of formation

·         contain transition metal ions in their higher oxidation states.

Progress will be most rapid in this area when the materials, whose properties are being studied are well characterized chemically and structurally so that measurements made in one laboratory can be compared in a meaningful manner with those obtained elsewhere.

1979

In the short term it is likely that improved lead-acid cells will be used to power the second generation road vehicles (milk floats being the first) that might reasonably be used by postal, telephone repair and metropolitan delivery fleets. In the long term, advanced batteries will probably utilize the alkali metals as anodes and will operate at or around ambient temperature. Present trends would seem to suggest  that a liquid electrolyte will be used in conjunction with a solid cathode. We are only now beginning to understand the phenomena of intercalation.

 

with A. J. Jacobson and R. R. Chianelli, ‘Amorphous molybdenum disulfide cathodes', Journal of the  Electrochemical Society, 126 (1979), 2277-2278

We report electrochemical data on amorphous MoS2 prepared at ambient temperatures by the reaction of LiS2 with molybdenum chloride. This synthetic technique gives materials with properties radically different from those commonly produced at high temperatures, such as a substantially larger capacity for electrochemical reaction with lithium and high reversibility

 

1980

The higher capacity of the amorphous over crystalline molybdenum disulfide is presumably associated with the disordered structure which prevents the decomposition to lithium sulfide found for the latter. Ongoing structural studies on the di- and tri-sulfides using EXAFS and radial distribution studies clearly show that there are marked differences between the amorphous and crystalline compounds.

The energy storage capacities of these amorphous cathode materials is very high. It remains to be proved whether they can cycle as well as TiS2.

Any transition metal in the van der Waals layer impedes intercalation by restricting the expansion of the crystalline lattice. Thus, the disulfides must not only be stoichiometric but also formed at sufficiently low temperatures to prevent the excitation of Ti ions into the layer. Sulfur pressure also plays an important role. The stoichiometric compound is best formed from titanium sponge at 450-600 degrees C in a temperature gradient that fixes the sulfur pressure. This TiS2 has been discharged more than 600 times.

 

The essential feature of the intercalation reaction, and that which makes its study so exciting and profitable, is that guest and host experience some degree, along a spectrum from subtle to extreme, of perturbation in their geometric, chemical, optical, and electronic properties. There is considerable latitude available to the worker for controlling many of the parameters in order to tailor the behavior desired.

TiS2 cells have been manufactured as small button cells for watches. However, before large lithium batteries can be made commercially available, safe high-rate electrolytes have to be found.

1987

"Sodium ion conduction in single crystal vermiculite",

The diffusion of sodium in single crystals of Llano vermiculite has been studied using conductivity measurements. The crystals were maintained in the bilayer water state by using aqueous contacts. The diffusion coefficient at 25°C was found to be 1.9 X 10^–8 cm²/s, with an enthalpy of motion of 11.7 kcals/mole. These values are in good agreement with sodium tracer and proton NMR studies and indicate that the sodium ions probably diffuse with their hydration spheres. The conductivity in the range 10 to 90°C is less than that in sodium beta alumina and much less than that of either surface clay cations or of aqueous sodium chloride solutions.

MSW, Chen R., T. Chirayil and P. Zavalij, “The intercalation and hydrothermal chemistry of solid electrodes”, Solid State Ionics, 94 (1997), 227-238

Intercalation chemistry plays a key role in the electrochemical reduction and oxidation by lithium of many solid electrodes, including transition metal compounds and graphite. For 25 years, from TiS2 and graphite to LixCoO2 and LiMn2O4, the electrode reactions of such intercalation compounds have been extensively studied. Much emphasis has been placed on two simple classes of structures, layer and spinel, both of which are formed by LixTiS2 and LixMnO2. More recently, much effort has been directed at synthesizing new structures that might show enhanced electrochemical activity. Soft chemistry approaches have been harnessed for this purpose. Mild hydrothermal reactions are one such approach. Several new vanadium oxides have been formed, as well as layered forms of manganese oxide. A number of these new compounds reversibly react with lithium and therefore may be used as the cathode in lithium batteries.

We have studied the hydrothermal synthesis of tetramethyl ammonium vanadium oxides that contain an additional transition metal, such as iron or zinc, in order to stabilize the layered structure of vanadium oxide for electrochemical redox reactions with lithium. Addition of the lower cost iron and zinc is also beneficial for commercial application. Several new layered structures which contain double sheets of V₂O₅ have been formed and characterized. The iron compound contains the TMA ion. Their structures and electrochemical behavior are reported.